Problem: $ B = \left[\begin{array}{rr}4 & 2 \\ 0 & 0 \\ 5 & 5\end{array}\right]$ $ D = \left[\begin{array}{rr}-2 & -2 \\ 0 & 4\end{array}\right]$ What is $ B D$ ?
Answer: Because $ B$ has dimensions $(3\times2)$ and $ D$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ B D = \left[\begin{array}{rr}{4} & {2} \\ {0} & {0} \\ \color{gray}{5} & \color{gray}{5}\end{array}\right] \left[\begin{array}{rr}{-2} & \color{#DF0030}{-2} \\ {0} & \color{#DF0030}{4}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rr}{4}\cdot{-2}+{2}\cdot{0} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{4}\cdot{-2}+{2}\cdot{0} & ? \\ {0}\cdot{-2}+{0}\cdot{0} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{4}\cdot{-2}+{2}\cdot{0} & {4}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{4} \\ {0}\cdot{-2}+{0}\cdot{0} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{4}\cdot{-2}+{2}\cdot{0} & {4}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{4} \\ {0}\cdot{-2}+{0}\cdot{0} & {0}\cdot\color{#DF0030}{-2}+{0}\cdot\color{#DF0030}{4} \\ \color{gray}{5}\cdot{-2}+\color{gray}{5}\cdot{0} & \color{gray}{5}\cdot\color{#DF0030}{-2}+\color{gray}{5}\cdot\color{#DF0030}{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}-8 & 0 \\ 0 & 0 \\ -10 & 10\end{array}\right] $